THIS invention relates to railway vehicle suspensions.
It is known that the wheelsets of railway vehicles which have live axles and wheels with conical or profiled treads are prone to excite oscillations of the vehicle in the lateral plane and such oscillations, often referred to as hunting, become unstable beyond a certain critical speed. For safe operation it is essential that this critical hunting speed is higher than the maximum operating speed of the vehicle and as operating speeds of trains have been steadily increasing in recent years novel railway vehicle suspensions are required to cope with this hunting problem.
An analysis of the hunting phenomenon shows that for the simplest railway vehicle or railway bogie which has two wheelsets the critical hunting speed decreases with increasing mass of the wheelsets and increases with increasing stiffness of the suspension elements which constrain the relative motions in the lateral plane of the two wheelsets, namely the yawing motions of the two wheelsets in an equal and an opposite sense of rotation and the relative lateral motions of the two wheelsets.
Conventionally the wheelset suspension consists of axle box springs and wheelset guidance elements which are elastic in the lateral and longitudinal directions. In this case the constraint to yawing motions of the two wheelsets in an equal sense of rotation and the constraint to relative lateral motions of the wheelsets (often referred to as shear stiffness) is generated by the combined in series elastic effect of the lateral and longitudinal stiffness of the elements which suspend the wheelsets to the bogie frame. The constraint to yawing motions of the two wheelsets in an opposite sense of rotation (often referred to as bending stiffness) is generated by the longitudinal stiffness of the elements which suspend the wheelsets to the bogie frame. Thus increases in shear and bending stiffness which, as mentioned above, will increase the critical speed of hunting, can be obtained by increasing the lateral and longitudinal stiffness of the elements which suspend the two wheelsets to the bogie frame. However, experience has shown that there is a limit to this as an increase in the stiffness of the wheelset suspension elements also causes the lateral and yaw oscillations of the bogie frame and the wheelsets to be strongly coupled dynamically and this has a de-stabilising effect on the vehicle.
In order to avoid this de-stabilising coupling effect between the bogie frame and wheelset oscillations it has been suggested to interconnect the wheelsets directly by means of lightweight, non-lead carrying members in order to obtain a shear and bending stiffness between the wheelsets which is independent of the longitudinal and lateral stiffness of the elements which suspend the wheelsets to the bogie frame. An example is described in the specification of U.S. Pat. No. 3,528,374 to Wickens.
Stiff interconnections, typically in the form of crossanchors or triangular frames joined at their apices to obtain a high shear stiffness have been applied particularly in the case of so-called self-steering or radial axle bogies which have a specified relatively low bending stiffness to allow the wheelsets to attain a radial position in curves, as exemplified by U.S. Pat. Nos. 4,067,261 and 4,067,262 to Scheffel. However, it has been found that for such wheelset interconnections to be effective the wheelsets have to be fitted with sturdy sub-frames that add to the mass of the wheelset and result in a de-stabilising effect which at least partially off-sets the gain in stability attributable to the elastic interconnection of the wheelsets.
Furthermore the application of known wheelset interconnections of cross-anchor or triangular frame type is limited to adjacent wheelsets. UK patent 1 508 194 to Wickens describes cross-anchor type interconnections between non-adjacent wheelsets, but teaches no practical method by which such interconnections can be achieved. Non-adjacent wheelsets are generally too far apart to allow for an effective wheelset interconnection of the known type to be fitted. However, an analysis of the hunting stability of multi-axle vehicles shows that the stability of the vehicle can be increased substantially if adjacent as well as non-adjacent wheelsets are interconnected with each other.
A further problem with known cross-anchor or triangular frame wheelset interconnections is that they cannot always be readily fitted due to space limitations. This applies particularly to motorised bogies and high speed bogies with elaborate brake gear.
As an alternative to the known cross-anchor or triangular frame interconnections it has been suggested to fit linkages between the wheelsets, which linkages are also attached to the bogie frame. See, for example, U.S. Pat. No. 3,862,606 to Scales, South African patent 86/0633 to Lukens General Industries Inc, and South African patent 82/6357 to Scheffel.
However, it has been found that such linkages do not improve the hunting stability of the bogie because the linkages do not only constrain the motions of the wheelsets in the lateral plane, but also the motions of the bogie frame. This causes the motions of the wheelsets and the motions of the bogie frame to be dynamically coupled, and such dynamic coupling negates the stabilising effect of the linkages.